The invention relates to a transducer for concurrently deriving a multichannel signal from a pair of modulation walls of a sound groove of a record disk, and more particularly to a transducer having a pair of moving coils which are located within the magnetic field established across a pair of pole pieces so as to correspond to the respective modulation walls of a sound groove.
British Pat. No. 1,000,035 discloses a transducer including a flat, square armature of a magnetic material around which a pair of moving coils are disposed in an orthogonal manner. With a transducer of this construction, since the coils share the armature, it is difficult to obtain high level of signal separation between the coils. Because the armature must carry the pair of coils, its volume is increased as is the radius around which the respective coils are disposed, with consequence that an oscillating system which comprises the armature and the pair of coils tends to have an increased value of mass. Hence, in order to reduce the equivalent mass, an arrangement must be employed such that most of the oscillating mass of the oscillating system be located on the center of oscillation. However, the resulting arrangement is considerably complex and requires a troublesome procedure for the manufacture and adjustment. Considering the dynamic aspect, the center of oscillation will undergo a slight displacement with the frequency of the oscillating system, so that the described arrangement is insufficient to achieve a reduction in the equivalent mass. As a consequence, the reproducible frequency range is limited in its high frequency region. A transducer of the type described is constructed such that the armature is mounted on the rear end of a cantilever arm and is supported together with the arm by a thin suspension rod with a damper of an elastic material disposed therebetween. However, because the rear surface of the armatures which is in contact with the damper is not flat as a result of the provision of the pair of coils thereon, the damping effect of the damper varies.
U.S. Pat. No. 3,679,843 proposes the use of an integral V-shaped armature having a pair of projections, instead of a square armature. Two sets of coils are disposed on each projection of the armature. In this design, although coils share a single armature as before, the volume of the armature can be decreased, individual coils can have a relatively greater number of turns, and a flat surface can be utilized for contact with the damper. However, as the projection of the armature is reduced in thickness, an undesirable resonance effect occurs as the stylus tip oscillates, thereby degrading the tone quality. For an industrial production of such transducer, it is necessary to use a press to form the V-shaped armature. However, the use of the press produces sharp edges or burrs, which may cause a short-circuiting of coils as the latter are disposed on the respective projections. Although it is possible to remove these sharp edges or burrs by a chemical treatment, this reduces the free end of the projections into a round form, reducing the length available for the disposition of a coil thereon. In addition, a difficulty is experienced during a coil winding operation since the armature cannot be rotated when winding a coil around each projection.